Isolation structure for semiconductor integrated circuit substrate

ABSTRACT

Isolation regions for semiconductor substrates include dielectric-filled trenches and field oxide regions. Protective caps of dielectric materials dissimilar from the dielectric materials in the main portions of the trenches and field oxide regions may be used to protect the structures from erosion during later process steps. The top surfaces of the isolation structures are coplanar with the surface of the substrate. Field doping regions may be formed beneath the field oxide regions. To meet the demands of different devices, the isolation structures may have varying widths and depths.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 11/298,075,filed Dec. 9, 2005, which is incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

This invention relates to semiconductor chip fabrication and inparticular to methods of fabricating structures to isolate electricallythe active or passive devices formed on a semiconductor chip.

BACKGROUND OF THE INVENTION

In the fabrication of semiconductor integrated circuit (IC) chips, it isfrequently necessary to electrically isolate devices that are formed onthe surface of the chip. There are various ways of doing this. A way isby using the well-known LOCOS (Local Oxidation Of Silicon) process,wherein the surface of the chip is masked with a relatively hardmaterial such as silicon nitride and a thick oxide layer is grownthermally in an opening in the mask. Another way is to etch a trench inthe silicon and then fill the trench with a dielectric material such assilicon oxide.

It is desirable to form these isolation structures early in the processbecause they can also act as barriers or stops to the lateral diffusionof dopants, thereby allowing a more closely packed device population onthe surface of the chip. In short, a dielectric-filled trench canfunction as a diffusion stop as well as an electrical isolationstructure.

The problem with forming a dielectric-filled trench early in the processis that subsequent process steps, which frequently include etching andcleaning, can etch or erode the dielectric material in the trench. Thiscan impair the value of the trench as an isolation structure and cancreate depressions in the top surface of the chip, rendering furtherprocessing more difficult.

This problem is illustrated in FIGS. 1A-1C. In FIG. 1A, a trench 101 hasbeen etched in a semiconductor substrate 100. In FIG. 1B, trench 101 hasbeen filled with a dielectric material 102 and the top surface has beenplanarized (e.g., by chemical-mechanical polishing) to form an isolationstructure. FIG. 1C shows the isolation structure after furtherprocessing, with part of the dielectric material 102 removed or erodedso as to form a recess or gap 103 on the top surface of the structure.Dielectric materials that are resistant to etching in normalsemiconductor processes (e.g., silicon nitride) tend to be hard,brittle, high-stress materials. When these materials are deposited in atrench they tend to crack.

A second problem stems from the fact that chips are generally dividedinto two general areas: broad or wide “field” areas and moredensely-packed device areas, sometimes referred to as “active” areas. Itis preferable to form relatively narrow, deep trenches in the activeareas to maintain a tight packing density and to form relatively widetrenches in the field areas to space out the devices over largerdistances. This creates a problem in filling the trenches. The narrowtrenches may be filled while the wide trenches are difficult to fill.Alternatively, using numerous narrow trenches to cover large distancesin the field areas can complicate the topography of the chip.

Accordingly, it would be desirable to develop a flexible, adaptabletechnique of forming dielectric-filled isolation structures that avoidsthe erosion of the dielectric fill material during subsequentprocessing. It would also be desirable to provide for the formation ofrelatively wide and narrow structures in the field and active regions,respectively, of the chip.

SUMMARY OF THE INVENTION

According to this invention, an isolation structure is formed by fillinga trench in a semiconductor substrate with a “dielectric fill.” Thedielectric fill includes a first dielectric material and a seconddielectric material. The first dielectric material is located in a lowerportion of the trench; the second dielectric material is located in anupper portion of the trench, the lower portion typically being larger inthe vertical dimension than the upper portion. The surface of the seconddielectric material is substantially coplanar with a surface of thesubstrate. The first and second dielectric materials are dissimilar inthe sense that the second dielectric material is not etched by achemical which etches the first dielectric material. Thus in subsequentprocessing the second dielectric material forms a protective cap overthe first dielectric material. Typically, the first dielectric materialis a relatively soft, low-stress material and the second dielectricmaterial is a relatively hard, etch-resistant material. Crackingproblems can be avoided by limiting the thickness of the seconddielectric layer to a value that provides protection during lateretching processes but does not create stress problems.

Alternatively, instead of forming a discrete cap, the trench may befilled with a “graded” dielectric, wherein the proportion of the seconddielectric material in the dielectric fill increases gradually as onemoves upward towards the mouth of the trench.

The sidewalls of the trench may be lined with an oxide layer to preventdopants from the dielectric fill from migrating into the semiconductorsubstrate.

In one group of embodiments the first dielectric material is a siliconoxide and silicate glass, either doped or undoped. The second dielectriccan be silicon nitride, a polyimide or any dielectric materialcontaining little or no silicon oxide.

The substrate may also include the lower portion of a field oxideregion, typically formed by a local oxidation of silicon (LOCOS)process. The surface of the field oxide region is also substantiallycoplanar with the surface of the substrate. Alternatively, a protectivecap may be formed over the field oxide.

In another group of embodiments, the substrate contains two isolationstructures, the first formed in a relatively shallow, wide trench, thesecond formed in a relatively narrow, deep trench. Both trenches arefilled with a dielectric fill and the surface of the dielectric fill issubstantially coplanar with the surface of the substrate. Alternatively,a protective cap of the kind described above may be formed at the mouthof each trench.

In yet another set of embodiments, one or more field oxide regions areformed in the same substrate as one or more trench isolation structures.Field doping regions of predetermined conductivity type and dopingconcentration may be formed under the field oxide regions. Optionally,protective dielectric caps may be formed where the trenches and fieldoxide regions meet the plane of the surface of the substrate. Thesurface of the entire structure is substantially coplanar. The surfacemab be planarized by using a chemical etchback, a plasma-enhanced orreactive ion etch (RIE), chemical-mechanical polishing (CMP) or somecombination thereof.

The invention also includes methods of fabricating isolation structures.One such method includes forming a trench in the semiconductorsubstrate; depositing a first dielectric material in the trench;removing a portion of the first dielectric material such that a surfaceof the first dielectric material is located at a first level below asecond level of a top surface of the substrate, thereby forming arecess; depositing a second dielectric material in the recess; andremoving a portion of the second dielectric material such that a surfaceof the second dielectric material is substantially coplanar with thesurface of the substrate, thereby forming a protective cap in thetrench.

Another method includes thermally forming a field oxide region at asurface of the semiconductor substrate; forming a trench in thesubstrate; depositing a first dielectric material in the trench;removing a portion of the first dielectric material such that a surfaceof the first dielectric material is located at a first level below asecond level of the surface of the substrate, thereby forming a recess;depositing a second dielectric material in the recess; and removingportions of the field oxide region and the second dielectric materialsuch that a surface of the field oxide region and a surface of thesecond dielectric material are substantially coplanar with the surfaceof the substrate, thereby forming a protective cap in the trench.

The methods of this invention are highly flexible and can be used toform isolation regions necessary meet the varying demands of differentregions and devices in a semiconductor substrate. The topography of thesubstrate is maintained extremely flat, or at least sufficiently flat asnot to interfere with or complicate the formation of fine line widthsand submicron features or the interconnection thereof during subsequentprocessing. Protective caps can be used to protect the dielectricmaterials from erosion during subsequent processing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C illustrate the problem that occurs when the dielectric in atrench which serves as an isolation structure is eroded during laterprocessing.

FIGS. 2A-2F illustrate a process for forming an isolation structure thatincludes a protective cap at the mouth of a dielectric-filled trench.

FIGS. 3A-3D illustrate a process for forming a modified version of theisolation structure of FIG. 2F where an oxide layer is formed on thewalls of the trench adjacent the protective cap.

FIG. 4 illustrates how the oxide layer shown in FIG. 3D may be eroded insubsequent processing.

FIG. 5 is a flow chart, in “card” form, of the processes shown in FIGS.2A-2F and 3A-3D.

FIGS. 6A-6C illustrate a process for forming isolation structures thatinclude a broad or wide field oxide region and a relatively narrowdielectric-filled trench.

FIGS. 7A-7H illustrate a processing for forming isolation structureswhich include a wide, shallow trench and a narrow, deep trench.

FIGS. 8A-8J illustrate another process for forming isolation structuresthat include a broad or wide field oxide region and a relatively narrowdielectric-filled trench, with a protective cap formed at the top ofeach structure.

FIGS. 9A-9E illustrate a process for forming isolation structures thatinclude a pair of field oxide regions and a dielectric-filled trench,with field doping regions beneath the field oxide regions.

DESCRIPTION OF THE INVENTION

FIGS. 2A-2F illustrate a process for fabricating a trench isolationstructure that avoids the formation of a gap or recess at the top of thetrench, as shown in FIG. 1C. As shown in FIG. 2A, an oxide or “hardmask” layer 121 is formed on the top surface of a semiconductorsubstrate 120, and a photoresist layer 122 is deposited on top of hardmask layer 121. The term “hard mask” is used herein to refer to athermally grown or deposited dielectric layer used as a mask during theetching of a trench in semiconductor substrate 120. The “hard mask” isdistinguished from the organic photoresist layer 122, for example, whichis mechanically softer and therefore subject to erosion during thetrench etch process. An opening is formed in photoresist layer 122 by anormal photolithographic process, and an opening 123 is etched in hardmask layer 121 through the opening in photoresist layer 122.

As shown in FIG. 2B, substrate 120 is etched through the opening 123 toform a trench 124. It is generally preferable to remove photoresistlayer 122 prior to the etching of the trench, since photoresist layer122 may interfere with the trench etching process, change shape duringthe trench etching process, and possibly introduce undesirable organiccontaminants into the trench. A reactive ion etch (RIE) can be used toachieve an anisotropic etch, producing a trench 124 having verticalwalls. A relatively thin oxide layer 125 is grown thermally on the wallsand floor of trench 124. If desired, a sacrificial oxide layer can beformed to remove crystal defects caused by the RIE process, thesacrificial oxide layer can be removed, and then a second oxide layercan be grown. The thickness of oxide layer 125 could be from 100 to 1000Å, typically about 300-400 Å. If trench 124 is later filled with a dopeddielectric material, oxide layer 125 will prevent the dopant fromentering the semiconductor material surrounding trench 124.

As shown in FIG. 2C, a relatively thick layer 126 of a glass such asborophosphosilicate glass (BPSG) is spun onto the surface of substrate120, completely filling trench 124. The BPSG could be doped to reduceits viscosity, or it could be undoped. Alternatively, the BPSG could bedeposited by chemical vapor deposition (CVD). As described above, if theBPSG layer 126 is doped, oxide layer 125 acts as a barrier to preventthe dopant from entering and doping substrate 120. BPSG layer 126 issufficiently thick (e.g., 0.5 to 1.0 μm thick) that its top surface isrelatively planar, with only a small dent over the location of trench124. If desired, a high temperature reflow can be used to furtherplanarize the surface of BPSG layer 126.

As shown in FIG. 2D, BPSG layer 126 and sidewall oxide layer 125 areetched back until their top surfaces are below the surface of substrate120, forming a recess 130. Following the etchback, the surface of BPSGlayer 126 may be from 0.1 to 0.5 μm (typically about 0.2 to 0.3 μm)below the surface of substrate 120. Then, as shown in FIG. 2E, a layer131 of another dielectric is deposited, filling recess 130 andoverflowing the surface of substrate 120. Layer 131 is then planarizedby CMP or etchback to form a protective cap 132, which completely coversand protects oxide layer 125 and BPSG layer 126. The top surface of cap132 is preferably coplanar with the surface of substrate 120, althoughit could vary by 0.1 μm in height across the wafer. FIG. 2F shows thestructure after cap 132 has been formed.

Layer 131 and cap 132 should be formed of a material that is notsignificantly etched by the cleaning and etching steps that are to takeplace later in the process. In this embodiment, for example, layer 131may be formed of silicon nitride. In general, the material of whichlayer 131 is formed does not etch at all, or etches substantially slowerthan BPSG layer 126 or oxide layer 125, in the subsequent processingsteps. A protective cap according to this invention can be formed at anytime during the process to protect the trench-fill material fromsubsequent erosion of the kind shown in FIG. 1C.

It should be noted that in general materials such as silicon nitridethat can provide a protective shield against further etching typicallydo not deposit very uniformly and thus it is difficult to get them tofill a trench. Moreover, silicon nitride tends to crack when depositedthickly. These problems are overcome by filling the trench with asofter, less brittle material such as BPSG and then covering thematerial with a relatively thin protective cap of a harder, more brittlematerial such as silicon nitride.

Table 1 shows the relative removal rates of materials that can be usedto fill the trench for several etchants or removal methods.

TABLE 1 Etchant or Removal Method Selective Selective Dielectric plasmaoxide “nitride” plasma Fill Materials 100:1 HF 10:1 HF etch etch CMPThermal SiO₂  30 Å/min  175 Å/min Ox: 500 Å/min Nit: 1200 Å/min Nit: <20Å/min Ox: 420 Å/min Spin-on glass (SOG) BPSG 1240 Å/min 7362 Å/min 8200Å/min 1800 Å/min Polyimide   5 Å/min   8 Å/min

There are numerous variations of the process illustrated in FIGS. 2A-2F.One such variation is shown in FIGS. 3A-3D. FIG. 3A is similar to FIG.2D and shows the structure after BPSG layer 126 and oxide layer 125 havebeen etched back until their top surfaces are below the surface ofsubstrate 120. As shown in FIG. 3B, a thin oxide layer 140 is thenthermally grown on the surface of substrate 120 and, as shown in FIG.3C, nitride layer 131 is then deposited. In this embodiment oxide layer140 separates nitride layer 131 from semiconductor substrate 120.Alternatively, an oxynitride layer may be deposited using chemical vapordeposition (CVD). When nitride layer 131 is planarized or etched back,as shown in FIG. 3D, the nitride cap that remains in the trench is notin contact with the sidewalls of the trench. While this cap may notprovide as effective a seal as the embodiment shown in FIG. 2F, thepresence of oxide (or oxynitride) layer 140 on the walls of the trenchtends to reduce the stress that is attributable to the different thermalexpansion coefficients of nitride and silicon, respectively. Oxide (oroxynitride) layer 140 thus provides stress relief.

Moreover, even if oxide layer 140 is over-etched to leave a small gap150, as shown in FIG. 4, gap 150 is nonetheless much smaller than therecess 103, shown in FIG. 1C, and much easier to fill with a subsequentlayer of, for example, BPSG. It is preferable, however, not to removeall of oxide layer 140.

FIG. 5 is a flow chart summarizing the processes described above, eachstep being represented by a “card” (the clipped cards denoting optionalsteps). In the first sequence the trench is formed by depositing a hardmask layer (e.g., oxide or nitride), depositing a photoresist layer,patterning the photoresist layer to create a trench mask, etching thehard mask layer through an opening in the trench mask, optionallyremoving the photoresist layer, and etching the trench through anopening in the hard mask layer.

In the next sequence, optionally a sacrificial oxide layer can be formedon the walls of the trench and removed, a lining oxide layer is grown,the trench is filled with a dielectric (e.g., BPSG), and optionally thedielectric can be planarized by etching or CMP.

Finally, the dielectric fill is etched back into the trench, optionallyan oxynitride or oxide layer is grown or deposited on the walls of thetrench, and a nitride layer is deposited and etched back until it issubstantially coplanar with the top surface of the substrate.

The examples above describe a structure wherein the surface of thesubstrate is essentially planar. A non-planar structure 200 isillustrated in FIG. 6A. A substrate 205 has a top surface 202. A trench201 has been etched in substrate 205 and a field oxide region 203 hasbeen thermally grown in the substrate such that field oxide regionextends upward beyond the surface 202 as well as downward into thesubstrate. A polysilicon layer 204 has been deposited on top of fieldoxide region 203. As is apparent, there is a considerable heightdifference between the bottom of trench 201 and the top of polysiliconlayer 204. If trench 201 is filled with a dielectric, an etchback can beused to planarize the surface of the dielectric with surface 202.Otherwise, if CMP is used to planarize the dielectric, it is clear thatpolysilicon layer 204 as well as a portion of field oxide region 203will be removed.

One solution to this problem is to omit the polysilicon 204 (or topostpone the formation of polysilicon 204 until later in the processflow) and to grow the field oxide region 203 thick enough that theportion below the surface 202 is sufficient to provide the necessaryelectrical characteristics. FIG. 6B shows trench 201 lined with an oxidelayer 206 and filled with BPSG 207, both of which have been etched backinto the trench. The entire structure is covered with a nitride layer208, which also fills the upper portion of the trench. In FIG. 6C, thetop surface has been planarized by CMP, leaving the bottom portion 209of field oxide region 203 and a protective nitride cap 210 over the BPSG207 and oxide layer 206. The top surface is totally flat. Since having anonplanar top surface greatly complicates further processing, the flatstructure shown in FIG. 6C is preferable to the structure shown in FIG.6A. Furthermore, since field oxide region 203 is grown by thermal means,the remaining region 209 can be very wide, whereas the trench can bevery narrow. To summarize, the structure shown in FIG. 6C includes a“capped” trench that is resistant to etching because of cap 210 and an“uncapped” field oxide region 209.

As an alternative, FIGS. 7A-7H illustrate a process by which a wideisolation trench and a narrow isolation trench can be formed using aminimal number of steps.

In FIG. 7A, a hard mask layer 252 has been deposited on a substrate 251,and a photoresist layer 253 has been deposited on top of hard mask layer252. Photoresist layer 253 is etched to form a wide opening and hardmask layer 252 is etched through the wide opening in photoresist layer253 to form a wide opening 254 which exposes the surface of substrate251.

As shown in FIG. 7B, substrate 251 is etched by RIE to form a widetrench 260. Photoresist layer 253 is removed and a new photoresist layer257 is deposited. If trench 260 is not too deep, photoresist layer 257will cover the step between the bottom of trench 260 and the top surfaceof substrate 251. A relatively narrow opening is etched in photoresistlayer 257, and hard mask layer 252 is etched through the opening inphotoresist layer 257 to form a narrow opening 256 which exposes thesurface of substrate 251. Alternatively, layer 257 may represent adeposited hard mask dielectric layer patterned and etched by aphotoresist layer (not shown).

As shown in FIG. 7C, substrate 251 is etched by RIE to form a narrowtrench 261. Photoresist (or hard mask) layer 257 and hard mask layer 252are then removed, or patterned and etched.

Optionally, a sacrificial oxide layer (not shown) can be grown intrenches 260 and 261 and removed to repair any crystal damage resultingfrom the RIE processes. As shown in FIG. 7D, a thin oxide layer 262 isgrown as a barrier against the diffusion of dopants into substrate 251,and a layer 263 of BPSG is deposited over the entire surface of thestructure. Alternatively, layer 263 could include any doped or undopedCVD-deposited or spin-on silicon oxide or silicate glass or any otherdielectric “fill” material, provided that the dielectric fill materialexhibits sufficiently low stress so as to avoid cracking duringsubsequent processing steps, during assembly, and during temperaturevariations encountered during device operation.

Of course, the process sequence could be revised such that the narrowertrench is formed before the wider trench.

Next, as shown in FIG. 7E, the entire top surface of the structure isplanarized by CMP or by a short chemical etchback followed by CMP.

Optionally, oxide layer 262 and BPSG layer 263 are etched back (e.g., byan acid or dry etch) into trenches 260 and 261 to form depressions 270and 271, as shown in FIG. 7F. A layer 280 of dielectric dissimilar tosilicon dioxide, silicate glass, or BPSG (e.g., nitride or polyimide) isdeposited over the top surface of the structure, as shown in FIG. 7G,and the top surface is again planarized to form protective caps 280 inthe mouths of trenches 260 and 261, shown in FIG. 7H. Unlike thedielectric fill material 263, the material used to form caps 280 maycomprise a brittle or high stress material, provided that the materialis not eroded by the normal etches encountered during subsequent waferprocessing in IC manufacturing and provided that caps 280 are madesufficiently thin to avoid cracking.

FIGS. 8A-8J illustrate a process for forming a capped isolation trenchand capped field oxide region. As shown in FIG. 8A, a pad oxide layer302 is grown on silicon substrate 301, and as in a typical localoxidation of silicon (LOCOS) sequence, a nitride layer 303 is depositedon pad oxide layer 302. Pad oxide layer can be 300 to 1000 Å thick, forexample. Nitride layer 303 is etched through a mask layer (not shown) toform a wide opening 304 which exposes pad oxide layer 302. As shown inFIG. 8B, the structure is heated (for example, to 900-1100° C. for 1 to4 hours) to form a thick field oxide region 305 in opening 304. As isnormal in a LOCOS process, nitride layer 303 is lifted up by theexpanding oxide at the edge of opening 304, forming the familiar “bird'sbeak” shape. Next, the remaining portion of nitride layer 303 is etched(FIG. 8C), and the top surface is planarized by a CMP process, yieldingthe result shown in FIG. 8D, with a smooth transition between theremaining portion 306 of field oxide region 305 and pad oxide layer 302.

Next, as shown in FIG. 8E, a photoresist layer 308 is deposited andpatterned to form a narrow opening 309. Oxide layer 307 is etchedthrough opening 309 and, as shown in FIG. 8F, substrate 301 is etched byan RIE process to form a narrow trench 310, with oxide layer 307 actingas a hard mask. The remains of oxide layer 307 may be removed in a shortcleaning step.

As shown in FIG. 8G, a thin oxide layer 311 is grown on the walls oftrench 310 and a layer 312 of BPSG or any other dielectric filler isdeposited. The top surface of substrate 301 is planarized by etching orCMP.

As shown in FIG. 8H, oxide layer 311 and BPSG layer 312 in trench 310and the remaining portion 306 of field oxide region 305 are etched backuntil the top surfaces of these elements are below the top surface ofsubstrate 301. A layer 315 of a dissimilar dielectric such as nitride isdeposited over the structure (FIG. 8I), and the structure is againsubjected to a CMP process to planarize the top surface and createprotective caps 316 over trench 310 and field oxide 306 (FIG. 8J).

FIGS. 9A-9E illustrate a process that produces a structure having fielddoping regions under the field oxide isolation regions but not under thetrench isolation structures.

In FIG. 9A, a pad oxide layer 351 has been grown on silicon substrate350, and a nitride layer 352 and photoresist layer 353 have beendeposited in that order on top of pad oxide layer 351. Photoresist layer353 is patterned to form two openings 354A and 354B, and nitride layer352 is etched through openings 354A and 354B to expose pad oxide layer351. Phosphorus (P+) is implanted through openings 354A and 354B to formN-type regions 356A. The dose of the phosphorus implant is typically inthe range of 5×10¹² to 3×10¹³ cm⁻² and the implant energy is typicallyfrom about 80 to 120 keV.

As shown in FIG. 9B, photoresist layer 353 is removed, and a newphotoresist layer 355 is deposited and patterned to form an opening thatincludes the location of former opening 354B in photoresist layer 353.Boron (B+) is implanted through the opening in photoresist layer 355 toform a P-type region 356B. Since the dose of the boron implant istypically an order of magnitude greater than the phosphorus implant(e.g., 8×10¹³ to 2×10¹⁴ cm⁻²) the boron counterdopes the phosphorusregion under opening 354B to form P-type region 356B. The energy of theboron implant is typically 60 to 120 keV.

Next, as shown in FIG. 9C, the structure is heated to form thick fieldoxide regions 370A and 370B in the locations of openings 354A and 354B.Field oxide regions 370A and 370B could be from 2000 Å to 2 μm inthickness (typically about 0.8 μm). This thermal process also activatesthe phosphorus and boron dopants and forms an N-type field doping region358A under field oxide region 370A and a P-type field doping region 358Bunder field oxide region 370B.

The remains of nitride layer 352 are removed (FIG. 9D), and optionally asacrificial oxidation may be preformed. Next, as shown in FIG. 9E, atrench 374 is etched and oxidized to form an oxide layer 371, followedby a dielectric fill with a material such as BPSG 372 in the mannerdescribed previously. The top surface of the structure is planarized byCMP or etchback, and oxide layer 371, BPSG 372 and the remains of fieldoxide regions 370A and 370B are etched back in the manner describedabove. A layer of nitride (or another dielectric dissimilar to thematerial used to fill trench 374) is deposited on the top surface, andthe surface is then planarized to form protective caps 373.

Alternatively, a layer of polyimide may be substituted for nitride layer352 and may be used to form the hard mask for etching trench 374.

This process yields a relatively narrow trench with no field dopingwhich might be used to isolate low-voltage devices, for example, andwide field oxide regions with field doping which might be used toisolate high-voltage CMOS devices, for example. The process gives thedesigner the ability to form isolation regions of different widths anddifferent field dopings in the same semiconductor substrate, with a flattop surface to simply any further processing. Moreover, the isolationregions can be formed with protective caps, if desired.

In some embodiments, the material in the trench is protected by a gradeddielectric fill in lieu of a discrete trench cap. In such embodiments,the trench is at least partially filled with a mixture of a relativelysoft, low stress dielectric and a relatively hard, etch resistantdielectric. The proportion of the relatively hard, etch resistantdielectric in the mixture increases as one approaches the mouth of thetrench. For example, a mixture of silicon dioxide and silicon nitridemay be deposited in the trench, with the percentage of silicon nitridein the mixture being increased near the mouth of the trench.

While specific embodiments of this invention have been described, itshould be understood that these embodiments are illustrative only, andnot limiting. Many additional or alternative embodiments in accordancewith the broad principles of this invention will be apparent to those ofskill in the art.

1. An isolation structure formed in a semiconductor substrate, theisolation structure comprising a trench formed in the substrate, thetrench comprising a mixture of a first dielectric material and a seconddielectric material, the proportion of the second dielectric material inthe mixture increasing with decreasing depth in the trench, wherein thesecond dielectric material is relatively more resistant to removal bynormal semiconductor etch processes as compared with the firstdielectric material.
 2. The isolation structure of claim 1 wherein thesecond dielectric material comprises silicon nitride.
 3. The isolationstructure of claim 2 wherein the first dielectric material comprisessilicon oxide.